The telecommunications industry is growing at an explosive pace as a result of the expanding need for the transmitting and receiving of greater amounts of information. The industry, in order to meet the needs of the market, has developed a number of technologies that make use of the inherent broadband capabilities of fiber optics. One of these technologies is Wavelength Division Multiplexing, or WDM.
WDM allows many signals to be transmitted simultaneously along a single optical fiber by sending each signal on a different carrier. Each carrier is a light beam of a slightly different wavelength than that of all of the other carriers. In order to combine these individual carrier beams into a single beam at the input of the fiber, an optical multiplexer (MUX) must be employed. To separate the carriers at the receiving end of the fiber, an optical de-multiplexer (DEMUX) must be employed. To be effective and economically practical, a MUX or a DEMUX must be capable of separating a multi-wavelength light beam into its individual wavelength components with a minimum amount of insertion loss and a minimum amount of Polarization Dependent Loss (PDL) and be relatively inexpensive and relatively compact.
The primary function of a DEMUX is to separate the carrier beams by wavelength. There are four basic means of providing this function: (1) thin film filters, (2) arrayed waveguides, (3) fiber Bragg gratings, (4) diffraction gratings. Thin film filters use multiple filters, each tuned to a different wavelength. Separation occurs at each filter along the light propagation path. This method is effective for systems with a small number of channels (one channel corresponds to one carrier wavelength). For systems with large numbers of channels (100 or more) thin film filters are not suitable because the insertion loss is excessive and the overall system becomes too complex.
Arrayed waveguides use an array of different length waveguides. A light beam consisting of multiple carriers, each at a different wavelength, exiting an input fiber is spread out so that it enters all of the waveguides In the array. The wavelength of each carrier in each waveguide, and the length of that waveguide, will determine its phase relative to the light of the same wavelength exiting all of the other waveguides. This phase relationship, in turn, will establish the overall phase distribution of the exiting wavefront for that particular wavelength. That phase distribution will then determine the output port to which this carrier wave will be directed.
Arrayed waveguides are very complex so that large arrays are difficult to make and some means of temperature control is generally required. This complexity places a practical upper limit on the number of channels that can be delivered with arrayed waveguides.
Fiber Bragg gratings are similar to thin film filters except that the filtering is done by a grating created within the fiber. The wavelength selection is done at each grating within the fiber. Fiber Bragg gratings have the same insertion loss problem as thin film filtersxe2x80x94the insertion loss becomes excessive for large numbers of channels and, as with thin film filters, the overall system becomes unacceptably complex for a large number of channels.
All three of the above technologies have a relatively high cost per channel.
The fourth technology, diffraction gratings, has the potential for both high performance (large number of channels and low insertion loss) and relatively low cost. A diffraction grating provides separation of a large number of discrete wavelengths by the process of dispersion. An incident beam consisting of multiple carriers of different wavelengths is dispersed by diffraction as the beam is either reflected from the grating or transmitted through the grating. Each wavelength of the exiting beam is reflected or transmitted at a different angle of diffraction so that each carrier can enter a different port. This would be the case for a DEMUX. For a MUX, the separate carriers would be combined into a single beam in a process that is essentially the reverse of that described above for a DEMUX.
The obvious advantage of diffraction gratings over the three other technologies is that a single, relatively simple device provides the complete wavelength separation function. Therefore, the cost, complexity and size of the MUX or DEMUX will all be less, yet the number of channels will be greater.
There are four types of diffraction gratings but only three are suitable for WDM applications reflective and transmissive surface relief gratings and transmissive volume phase gratings. Surface relief gratings can have relatively high diffraction efficiencies, but generally only for one polarization. This creates a problem in WDM that is known as Polarization Dependent Loss, or PDL. While PDL cannot be eliminated in a surface relief grating, it can be minimized, but only at relatively low grating frequencies (roughly 600 lines per mm or less). This low grating frequency reduces the dispersion of the grating, making it more difficult to insert more channels and get good channel separation.
Transmissive volume phase gratings can also have high diffraction efficiencies but, as in the case of surface relief gratings, this high diffraction efficiency generally occurs only for one polarization. Therefore, a conventional volume phase grating (VPG) will also exhibit high PDL. While PDL can be minimized in a conventional VPG, either the overall diffraction efficiency will be low or the dispersion will be low, resulting in either unacceptably high insertion loss or relatively fewer channels.
It is obvious that none of the aforementioned technologies provides what is desirable in a WDM devicexe2x80x94large number of channels, low insertion loss and low PDL across the full bandwidth of one of the telecommunications bands.
The present invention provides a means for overcoming the shortcomings of the prior art in WDM devices by creating an Enhanced Volume Phase Grating (E-VPG) that has high dispersion and uniformly high diffraction efficiency across a broad wavelength range for all polarizations. In the E-VPG, the thickness of the volume phase grating material, its bulk index, its index modulation and the angles of incidence and diffraction are all established so that the diffraction efficiency for both S-polarization and P-polarization are simultaneously maximized at the nominal wavelength. The volume phase grating material is created, coated, exposed and processed so that the desired values of these four major parameters are obtained.
A set of equations is developed that will determine these desired parameters for a number of different possible grating designs. Gratings in air or gratings attached to one or two prisms (a design often referred to as a Carpenter""s prism or grism) can be created using these design equations. Some of the designs will have higher dispersion than others but all will have high dispersion and high diffraction efficiency across a relatively wide bandwidth for all polarizations.
By employing a reflective element in the path of the diffracted beam, the E-VPG can be used in a reflective, double pass mode so that the overall dispersion is increased over that of a single E-VPG, while still maintaining high overall diffraction efficiency and low PDL.
The advantage of the present invention over the prior art is that it can provide the advantages of diffraction gratings over the three competitive technologiesxe2x80x94thin film filters, arrayed waveguides, fiber Bragg gratingsxe2x80x94without the disadvantages generally associated with conventional volume phase gratings or surface relief gratingsxe2x80x94high PDL and/or low dispersion and/or high insertion loss.